The present invention relates to an adaptive predictive processing system which encodes, for instance, a speech signal or musical signal, into predictive codes adaptively changing its coding characteristics in accordance with its property, or adaptively decodes the speech signal or musical signal from such encoded codes.
A conventional adaptive predictive coding system is one that encodes the difference (a prediction residual) between a value linearly predicted from several previous sample values of an input signal and a current input sample value and transmits the encoded difference. Various modifications can be effected according to the arrangements of a predictor and a quantizer employed. For an unsteady input signal, such as a speech signal, it has been proposed to improve the signal-to-quantization noise ratio (SN ratio) by changing the prediction coefficient and the quantization step size to comply with the statistic property of the signal. With the prior art adaptive predictive coding system, however, a bit rate above 32 Kb/s is needed for achieving quality equal to the toll quality and, at the bit rates below 16 Kb/s, the SN ratio is abruptly degraded and the quality is also markedly deteriorated by the quantization noise. Accordingly, the conventional system is not satisfactory.
An adaptive predictive coding system with adaptive bit allocation has been proposed as a system for improving such performance degradation in the adaptive predictive coding at low bit rates (see, for example, Japanese Patent Application. No. 42858/79, Pat. Disc. No. 13541/80 or Proc. of 1982 IEEE ICASSP, pp 1672-1675, May 1982, "Adaptive Bit Allocation Scheme Predictive Coding of Speech"). According to this system, the quantization bit number and the quantization step size of a quantizer are adapted in accordance with non-uniformness in both of a frequency domain and a time domain of residual power, and the prediction coefficient is computed from an autocorrelation coefficient of the input signal using a linear predictive analysis and adapted for each short time interval. Since this conventional system involves the computation of the autocorrelation and the residual power for the adaptation of the quantization bit number, the quantization step size and the prediction coefficient, the word length representing these quantities has to be about twice the word length of the input signal in the case of implementing the system and a high-speed multiplier is required, resulting in the scale of hardware inevitably becoming large. Moreover, this system calls for transmission of the prediction coefficient to the receiving side separately of the residual signal.